1. Field of the Invention
The present invention relates to a method of varying the ink density of the full tone in offset printing within a rotary printing machine.
2. Description of the Related Art
In digital printing processes, i.e., processes for producing printing plates in a binary sense, an ink supply is either accepted locally or not. This is the case, for example, in planographic printing or offset printing. The ink density of uninterrupted ink layers is the full tone characteristic of the ink layer and is controlled by the rate of the ink supply from the ink supply system to the printing plate.
In conventional offset printing, the ink supply and therefore the thickness of the ink layer supplied to the printing plate is regulated via inking zone screws. The printing plate has ink-accepting and ink rejecting regions and picks up the ink in proportion to the amount supplied only in those regions that are ink-accepting. The amount of ink picked up is also dependent on the ink splitting which occurs. A higher supply of ink from the ink supply system produces a higher ink layer density and therefore a higher full-tone density.
However, the ability of the inking unit to regulate the ink supply has disadvantages both with regard to the expenditure on control and also with regard to the complexity of the inking unit which results from this. The regulation of the ink supply also has a disadvantage with regard to the desired freedom of reaction of various ink uptake rates on following printed copies.
To reduce these disadvantages, short-form inking units such as the Anilox inking unit in offset printing have been developed for printing with low-viscosity printing inks for newspaper printing, for example, which bring the ink more directly onto the printing plate via an engrave roll and few intermediate cylinders. These short-form inking units therefore have a considerably reduced complexity with all the advantages which result from this. However, this form of the inking unit permits only very restricted regulation of the ink supply.
Each printing material needs a specific quantity of ink for a defined full-tone density, depending on the surface roughness, absorbency, ink absorption and so on. An inking unit on which the quantity of ink cannot be regulated in connection with a binary printing plate can therefore implement only specific full-tone densities, which fluctuate in accordance with the type of printing material, but the intention is not for a different engrave roll or an ink with a different pigment concentration or viscosity to be used depending on the printing material.
It is an object of the present invention to develop a method of varying the ink density of the full tone in printing within a rotary printing machine which, in spite of a constant ink supply from the inking unit or the ink-applying elements, permits control of the full-tone density or adaptation to the raster tonal values in the print.
The object of the present invention is achieved by a method of varying the ink density of the full tone in printing within a rotary printing machine with an ink application system that provides a constant quantity of ink which includes the steps of (1) setting a binary image on a printing plate in which a basic raster of raster points for the variable area image information is produced on the printing plate and determines the area coverage and (2) superimposing the basic raster on a fine micro raster such that the area coverage of the basic raster is reduced by a percentage within the rage including 0% to 100%. The printing process itself used in this case may, for example, be lithographic offset printing, relief printing, flexographic printing, electrophotographic printing, or electrographic printing. However, the invention is not restricted to these processes.
The geometric tonal value gain when ink is transferred from the printing plate to the printing material is taken into account according to the present invention. The term tonal value gain is based on the term area coverage. Area coverage is defined as the proportion of the area at a specific location which is covered with ink. The area coverage may be measured using optical geometrical measurement methods which measure the pure geometrical area coverage or by the measurement of the transmission relationships of a fully covered area (full tone) and the partially covered area (half tone), which then measure the effective or optical area coverage.
In addition to the full-tone density and therefore the ink layer thickness, the raster point size (in a basic raster) is a critical factor for the print quality. Brighter ink nuances are normally represented in the print by rastering these three primary colours, cyan, magenta and yellow together with black. During the setting of the binary image on the printing plate, the raster point size is defined in accordance with the tonal values of the respective image information. During the rastering process, bright image points are broken down into small raster points and dark image points are broken down into larger raster points (binary, variable-area image information). This applies both to a periodic, autotypical raster and a stochastic raster.
To register and define the various items of binary image information in numeric terms, use is made of the area coverage in percent. A raster tonal value can be specified in percentage of area coverage, that is to say 0% for white and 100% for a solid area. However, as is known, the raster tonal value in the print does not correspond to the geometric area coverage on the printing plate because both geometric and optical effects produce tonal value gain.
The term xe2x80x9ctonal value gainxe2x80x9d as used herein is therefore the increase in the area coverage from the printing plate to the printed material. The tonal value gain breaks down into two components, i.e., an optical one and a geometric one. The optical component is brought about by immigration of light in the printing material (light capture) from the uncovered areas to the covered areas. The geometric component, which is relevant especially for plate the method according to the invention, is brought about as a result of squeezing effects at the ink transfer points from the printing plate to the printing material or, in electrophotography, by tonal clouds around the actual image points. As a result of this effect, the area on the printing plate not covered by ink, i.e., the uncovered area, is reduced geometrically from the edges of the covered area during transfer of the ink to the printing material.
To control the quantity of ink transferred to the printing material with a constant supply of ink, the basic raster of raster points for the variable-area image information, which determines the area coverage, is superimposed on a very fine microraster which reduces the area coverage of the basic raster by a set percentage. The microraster is preferably finer by at least a factor of two than the basic raster. Then, in accordance with the geometrically covered areas defined by the basic raster and the microraster, the printing plate picks up ink from the system that provides the ink in offset these are the applicator rolls of the inking unit. However, the microraster does not appear on the printed material because of the effect of the tonal value gain, which results from the difference between the known raster tonal value for setting an image on the printing plate and the measured raster tonal value in the print. The tonal value gain as a deviation of the raster tonal value in the print from the raster tonal value of the printing plate can be represented in a print characteristic so that it can be used directly for setting an image and placing the set image on a microraster. The creation of a characteristic based on the tonal value gain and its use in printing process is sufficiently well known from the densitometric measurement techniques for printing machines and is not explained further here.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.